It's a very common thing in Linux, but coming from Windows this was new to me:
select something, and the mere fact that it was selected/highlighted is enough
to make it available for "paste" using the middle-click. It's a common thing
that should work pretty much everywhere in Linux it seems, and once you're used
to it, it can be quite useful.

Firefox, my browser of choice for a few years now, naturally supports this.
However, its behavior in that matter is regulated through a couple of options,
and their default values aren't the same on Windows and Linux. On linux, both
middlemouse.paste and middlemouse.contentLoadURL default to true (while on
Windows both default to false).

The second one is especially nice, because it means that if you do have an URL
ready to be middle-pasted, a middle click anywhere on the current page will
automatically open said URL. Pretty nice, however there were a couple of things
I wanted:

I wanted said URL to be opened in a new tab

I thought it would be nice if, when I have something other than a URL,
instead of doing nothing it would start a search

Middle Search

The second issue was solved thanks to a nice FF extension, Middle
Search by Ivo Danihelka. This extension does exactly that: if the text
is not an URL (does not start with http(s):// or ftp://) then it will be
used as keyword in a new search, done using your default search engine.

The only little problem I had with it, is that - much like the native handling
of the URL - it happens in the current tab, when I'd like it to open a new tab
instead.

Fortunately this FF extension is made of a little JavaScript, and could be
easily modified. Even better, I could also make it take care of the case when an
URL is present, instead of letting Firefox deal with it, and therefore open it
in a new tab as well.

If you also want a version that opens everything (URL and searches) in a new
tab, the modified source code is available on this BitBucket
repository. You can also find an XPI
file of said version.

Edit 2011-12-21: updated download link to v0.8.1.1; just a bump in version
numbers to support Firefox 9.

Selection in the address bar

One last thing about this whole issue, and again a difference in how Firefox
behaves (by default) depending on your OS. On Windows, a click in the address
bar will automatically select it all. Not so in Linux, which isn't a bad idea
considering how the whole middle-paste thing works.

However, on Windows when you double-click somewhere on the URL (in the address
bar), only part of it gets selected. It works on a "word" basis, which makes it
pretty easy to quickly select only a part of the URL, for example only the
domain name, or the file name. But in Linux, the double-click will select the
whole thing!

It annoyed me, but it doesn't anymore since, once again, this is only one
setting that can be easily changed. Open about:config and search for
browser.urlbar.doubleClickSelectsAll What it does is pretty obvious, and it
does default to false on Windows, but true on Linux. Once adjusted, the
double-click will work again on a "per-word" basis, and you will still be able
to select the whole URL with a triple-click.